In the metalworking industry, power presses are often used to form stock material such as steel or sheet metal into a variety of components. For example, in the automotive industry, sheet stock is formed into components of relatively small parts, such as engine struts, as well as significantly larger vehicle body components, such as deck lids, doors, and quarter panels. In these operations, the work piece is typically drawn or struck one or more times between upper and lower die halves to form the stock into a desired shape. Due to the particular shape of the article, in many applications it is necessary to perform an operation on the work piece at an angle other than with the travel of the press. For example, it may be necessary to punch an opening into or trim flash from the edge of the work piece.
Cam units are often used in power presses to perform these auxiliary tooling operations such as punching, forming, re-striking, flanging and trimming. An example of such a cam unit can be seen in co-pending application Ser. No. 07/819,347 now U.S. Pat. No. 5,269,167. As shown in that application, the cam unit is equipped with a tool mounted on a slide block and is installed on either the lower die or the upper die of the power press. The cam is constructed such that as the power press cycles, the cam unit compresses in a manner that converts the vertical motion of the press into a lateral motion of the tool mounted on the cam. This lateral movement brings the tool into contact with the work piece and thereby provides the force required to perform the tooling operation.
Cam units used in tooling operations typically require springs that will provide both the force needed to move a slide block with an attached tool into home position and provide the force needed to perform the desired stripping/work holding operation. However, the force required to perform the stripping/work holding operation is usually much greater than the force required to move the slide block. Most conventional springs are limited in the amount of force they exert. Therefore, to attain the desired stripping/work holding forces, conventional springs would have to be used in such numbers and be preloaded to such a degree that they would prove ill suited for most cam designs. Moreover, the force build-up to the slide block while being placed in position to do work would then be unnecessarily excessive and thereby abuse the mechanism and waste press energy.
Traditionally, gas springs such as nitrogen springs have been the spring of choice for cam operations. The use of these gas springs in a cam unit is illustrated by reference numeral 128 in FIG. 3 of co-pending application Ser. No. 07/819,347 now U.S. Pat. No. 5,269,167. These springs supply the high forces needed to perform stripping/work holding operations but, as mentioned above, they supply these same high forces throughout the entire slide block movement. Gas springs require an elaborate support system of compression chambers, tubing and valves to control the amount of gas pressure in the gas springs. This support system wastes valuable space in production lines and requires maintenance and frequent monitoring by operating personnel. Furthermore, since the gas pressure is manually controlled the effectiveness of the gas springs is dependent upon the activities of the operating personnel. Consequently, human error in the monitoring of the gas springs can result in poorly tooled work products.
Thus, the prior art has failed to provide a low cost spring for cam operations that provides sufficient force for tooling operations without supplying excessive force to the slide block. Consequently, there is a long felt need for a compact, low cost, self-monitoring spring that supplies large forces with short axial compression distances during tooling operations and low forces and large axial compression distances during the rest of the cam operations.